A spark plug is attached to an internal combustion engine (engine), etc., and is used to ignite a fuel-air mixture within a combustion chamber or the like. In general, such a spark plug includes an insulator having an axial hole extending in an axial direction; a center electrode inserted into a forward end portion of the axial hole; a metallic shell provided around the insulator; and a ground electrode fixed to a forward end portion of the metallic shell. A high voltage is applied to a gap formed between a distal end portion of the ground electrode and a forward end portion of the center electrode, whereby spark discharge occurs, and the fuel-air mixture or the like is ignited.
Incidentally, when the size of the above-mentioned gap increases as a result of consumption (erosion) of the center electrode caused by spark discharge, etc., the voltage required to generate spark discharge (discharge voltage) also increases. If the discharge voltage becomes excessively high, generation of spark discharge becomes impossible (so-called misfire occurs).
A conceivable method of overcoming such a drawback is enhancing durability (erosion resistance) of the center electrode by joining to a forward end portion of the center electrode a tip which is formed of a metallic material which is excellent in erosion resistance (e.g., iridium or platinum) and which has a relatively large diameter. When such a tip is employed, the diameter of the forward end portion of the center electrode may be increased such that the diameter of the forward end portion of the center electrode becomes equal to the diameter of the tip. However, in the case where the coefficient of linear thermal expansion of the center electrode is larger than that of the tip (for example, in the case where the center electrode is formed of a nickel alloy or the like, and the tip is formed of an iridium alloy or the like), if the diameter of the forward end portion of the center electrode is increased, a very large thermal stress acts on the joint interface between the center electrode and the tip under a high temperature condition (e.g., when an internal combustion engine is operated). As a result, formation of a crack at the joint interface becomes more likely, and the tip may come off the center electrode.
A conceivable method of solving such a problem is rendering the diameter of the forward end portion of the center electrode smaller than that of the tip (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-83662, etc.).